The ultrastructure of the intestinal nerve of Remak in the domestic fowl

Summary An ultrastructural study was made of the neurons, satellite cells and vesiculated axons of the intestinal nerve of the domestic fowl. Broad membrane-to-membrane contacts between adjacent nerve cell bodies were sometimes observed. The cell bodies and processes were not always separated from the extracellular space by a capsule of satellite cells. Following fixation using potassium permanganate, catecholamine (CA)-containing neurons in the intestinal nerve, unlike those in the lumbar parasympathetic ganglia, did not possess any small granular vesicles (SGV). Following exposure to noradrenaline, SGV could be demonstrated in the cell bodies of the juxta-ileal ganglia but not the juxta-rectal ganglia of the intestinal nerve. Non-CA axons were examined in tissue from birds that had been pretreated with 6-hydroxydopamine. Approximately one half of the non-CA axons formed axo-somatic contacts. Most of the non-CA axons contained varying proportions of small clear vesicles, large clear vesicles and large granular vescles. Statistical analysis showed that the non-CA axons could not be subdivided according to their vesicle content. CA-axons contained many SGV and were found in close apposition to neuronal somata and processes, and in the neuropil.     

Ultrastructure of murine cardiac ganglia in experimental Chagas' disease.

Albino mice, infected with Trypanosoma cruzi (Tulahuen strain) were sacrificed on days 7, 9, 12, 14, 16, 18, 21, 32 and 39 following infection. Transmission electron microscopic examination of the cardiac ganglia revealed no ultrastructural change at day 7. At day 9 there was peri- and intraganglionic monocytic infiltration but parasites were absent. Between days 12 and 16 there was intense monocytic infiltration, with intra-ganglionic presence of parasites within fibroblasts, monocytes and macrophages. None were seen within capsular cells, endothelial cells, Schwann cells, satellite cells and ganglion cells. The Schwann cells and satellite cells, however, showed phagocytic activity. Satellite cells were also reactive with proliferative pseudopodia which encircled neuronal processes. By day 18, parasites were absent in the ganglia. But monocytes were still present up to day 39, some of them still engulfing satellite cell and neuronal processes. Satellite cells continued to be reactive and Schwann cells phagocytic. Ganglion cells remained normal throughout the experiment. The results suggest that infection of Schwann cells, satellite cells and ganglion cells may depend upon the tissue tropism of the strain of the parasite used and its concentration in the inoculum. The results are consistent with the view that any parasympathetic dysfunction in experimental Chaga's disease in the mouse may be of a transient nature.     

Scanning electron microscopy of the muscle coat of the guinea-pig small intestine

Summary We have studied the layers of the muscular coat of the guinea-pig small intestine after enzymatic and chemical removal of extracellular connective tissue. The cells of the longitudinal muscle layer are wider, have rougher surfaces, more finger-like processes and more complex terminations, but fewer intercellular junctions than cells in the circular muscle layer. A special layer of wide, flat cells with a dense innervation exists at the inner margin of the circular muscle layer, facing the submucosa. The ganglia of the myenteric and submucosal plexuses are covered by a smooth basal lamina, a delicate feltwork of collagen fibrils, and innumerable connective tissue cells. The neuronal and glial cell processes at the surface of ganglia form an interlocking mosaic, which is loosely packed in newborn and young animals, but becomes tightly packed in adults. The arrangement of glial cells becomes progressively looser along finer nerve bundles. Single varicose nerve fibres are rarely exposed, but multiaxonal bundles are common. Fibroblast-like cells of characteristic shape and orientation are found in the serosa; around nerve ganglia; in the intermuscular connective tissue layer and in the circular muscle, where they bridge nerve bundles and muscle cells; at the submucosal face of the special, flattened inner circular muscle layer; and in the submucosa. Some of these fibroblast like cells correspond to interstitial cells of Cajal. Other structures readily visualized by scanning electron microscopy are blood and lymphatic vessels and their periendothelial cells. The relationship of cellular elements to connective tissue was studied with three different preparative procedures: (1) freeze-cracked specimens of intact, undigested intestine; (2) stretch preparations of longitudinal muscle with adhering myenteric plexus; (3) sheets of submucosal collagen bundles from which all cellular elements had been removed by prolonged detergent extraction.     

The ultrastructure of paraganglia associated with the inferior mesenteric ganglia in the guinea-pig

Summary The paraganglia of the inferior mesenteric ganglia in the guinea-pig are composed of small chromaffin cells containing an abundance of granule-containing vesicles. The chromaffin cells are almost completely surrounded by satellite cells. In areas in which satellite cell processes do not intervene, the membranes of adjacent chromaffin cells are closely apposed and often form specialized attachment zones. The paraganglia contain a dense capillary network, the endothelial cells of which are often extremely attenuated and show areas of fenestration. The processes of chromaffin cells approach close to the capillary walls and are often bare of satellite cells covering on the side facing the capillary. Evidence has been obtained for the exocytotic release of the contents of chromaffin cell vesicles into pericapillary spaces. Synapses of cholinergic and noradrenergic axons are seen on the chromaffin cells. The cholinergic axons degenerate when the praganglia are decentralized, but the noradrenergic axons, which appear to arise from the local inferior mesenteric ganglia, remain intact. The results suggest that the paraganglia have an endocrine function.     

Melanogenesis in cultures of peripheral nervous tissue: I. The origin and prospective fate of cells giving rise to melanocytes

Chick embryo spinal ganglia, peripheral nerves, and connective tissue usually associated with ganglia were cultured separately using several combinations of media and substrata. Melanocytes appear in cultures of both ganglia and peripheral nerves. The only cell type common to both the ganglion and peripheral nerve that could account for the observed pigment cells was the population of small cells with intensely staining nuclei that normally associates closely with nerve cell bodies and fibers. These cells could be distinguished morphologically from fibroblastic cells, which originated in the connective tissue capsule and did not undergo melanogenesis. We conclude that these small cells are supportive (Schwann, satellite, and perineurial) cell precursors and are one source of melanocytes in cultured peripheral nervous tissue.     

Light and electron microscopic histochemistry of the monoamines in the human foetal sympathetic ganglion in culture

Synopsis Sympathetic ganglia of 13 to 19-week-old human foetuses were cultured in small pieces with and without nerve growth factor for up to 5 weeksin vitro. The cultures were studied using phase-contrast, fluorescence and electron microscopy. Monoamines were demonstrated with the formaldehyde-induced fluorescence method, with and without pretreatment of the cultures with catecholamines or monoamine oxidase inhibitor.In the long-term cultures, primitive sympathetic cells, sympathicoblasts of types I and II, and young sympathetic neurons showed a fine structure identical to that described earlierin vivo. There were virtually no satellite or Schwann cells in the cultures. The neurons showed a considerable capacity to grow new nerve fibres in culture, even without nerve growth factor. Nerve terminals with accumulations of other nervous structures. Large granular vesicles were regularly found in the sympathicoblasts after glutaraldehyde-osmium tetroxide fixation. After permanganate fixation, dense-cored vesicles typical of adrenergic neurons were not seen, either in the perikarya, or in the processes, although it was possible to demonstrate specific fluorescence. No small intensely fluorescent (SIF) cells were observed.Variable formaldehyde-induced fluorescence was observed in the nerve cell perikarya and nerve fibres. The intensity of the fluorescence increased after treatment of the cultures with monoamine oxidase inhibitor and after incubation with catecholamines.     

Prenatal development of the rat dorsal root ganglia

Summary This study describes three-dimensional aspects of the development and pseudo-unipolarization of neuroblasts and the maturation of satellite cells in prenatal rat dorsal root ganglia, using scanning electron microscopy, after removal of extracellular connective tissue components by trypsin digestion and HC1 hydrolysis.At 14 days of gestation, the vast majority of neurons are spindle-shaped or bipolar and only 3% are unipolar, while at 16 and 18 days this percentage has increased to 30% and 91%, respectively. The initial portions of the central and peripheral neuronal processes gradually approach each other and form a common initial portion. Finally, the cytoplasm of this common initial portion becomes thinner and elongates to form the stem process of the mature cell.Satellite cells are present from the beginning of the period studied, but intricate networks of branching satellite cell processes only develop after about day 17.     

Contribution to ontogenesis of Merkel cells

Merkel cells appear in the epidermis of planum nasale of the rat fetuses from the 16th day of i. u. development, namely in the 2nd-3rd layer of epidermal cells. Nerve fibres appear in the subepidermal connective tissue from the 20th day of i.u. development. Long cytoplasmic processes filled in with specific dense core vesicles grow from Merkel cells against them. Intraepidermally, nerve fibres appear in postnatal period (from 3rd day after birth). Granular vesicles of Merkel cells probably have the leading role in the formation and maintenance of contacts between Merkel cell and the nerve ending. The results of studying ontogenetic development of Merkel cells in the rat are favour of hypothesis about the differentiation of Merkel cells in the epidermis, however, the possibility of secondary equipment of epidermis with Merkel cells independently on the development of nerve fibres is not eliminated.     

Clusters of nerve cell bodies enclosed within a common connective tissue envelope in the spinal ganglia of the lizard and rat

Summary A careful search for groups of nerve cell bodies enclosed within a common connective envelope was made in the spinal ganglia of the lizard and rat using a serial-section technique. Nerve cell bodies sharing a common connective envelope were found to be more common in the lizard (9.4%) than in the rat (5.6%). These nerve cell bodies were arranged in pairs, or, less frequently, in groups of three. At times, they appeared to be in immediate contact, with no intervening satellite cells; at other, they remained separated from one another by a satellite cell sheet. The clusters of nerve cell bodies enclosed within a common connective envelope probably result from the arrest of developmental processes in the spinal ganglion. It is possible that, as a result of the cell arrangement here described, certain neurons electrically influence other sensory neurons at the level of the ganglion.     

Ultrastructure of the nerve cells and fibres in the urinary bladder wall of the cat.

The nerve elements in the urinary bladder of the cat were studied by electron microscopy. According to their ultrastructure, nerve cell somata can be classified into three types: the large cells with a cytoplasm rich in organelles, several processes and numerous synaptic contacts on their surface; the cytoplasm contained 80- 120-nm granulated vesicles. The second type is poor in cytoplasmic organelles and has very few processes and virtually no synaptic contacts on the soma. The third type contains numerous large 160- to 220-nm 'neurosecretory' vesicles in the cytoplasm. According to the morphology of the vesicle population, four types of nerve processes could be distinguished: Type a, with a dominant population of small (40-60 nm) agranular vesicles. These are thought to be sacral parasympathetic fibres. Type b, with small (40-60 nm) granular vesicles, which may be the noradrenergic sympathetic fibres. Type c, with 80- to 120-nm granulated vesicles, probably of local origin. Typed d, with large 160- to 220-nm 'neurosecretory' vesicles also of local origin. Different types of nerve fibres are converging on the local nerve cells. This suggests that the local circuits can play an important role in coordinating the function of the bladder. Therefore, ganglia may be considered as an elementary functional unit.     

Evidence that myenteric neurons of the gastric corpus project to both the mucosa and the external muscle: myectomy operations on the canine stomach

Summary The distribution of nerve cell bodies and fibres in the canine stomach was investigated using antibodies to the general neuronal marker, neuron-specific enolase. Prominent ganglia containing many reactive nerve cells were found in the myenteric plexus of the gastric corpus and antrum. Nerve cells were absent from the submucosa of the corpus and were extremely rare in the antrum. Renoval of areas of longitudinal muscle and myenteric plexus from the corpus (myectomy), with 7 days allowed for axon degeneration, resulted in the loss of fibres reactive for galanin, gastrin-releasing peptide, substance P and vasoactive intestinal peptide from both the circular muscle and mucosa in the area covered by the lesion. Combined vagotomy and sympathetic denervation did not significantly affect these fibres, but did cause fibres reactive for calcitonin gene-related peptide to degenerate. It is concluded that the myenteric plexus of the gastric corpus, like the myenteric plexus of the small intestine and colon, is the source of nerve fibres innervating the circular muscle, but, in contrast to other regions of the gastrointestinal tract, myenteric ganglia, not submucous ganglia, are the major, or sole, source of the intrinsic innervation of the mucosa.     

In vivo transplantation of mammalian neural crest cells into chick hosts reveals a new autonomic sublineage restriction.

The study of mammalian neural crest development has been limited by the lack of an accessible system for in vivo transplantation of these cells. We have developed a novel transplantation system to study lineage restriction in the rodent neural crest. Migratory rat neural crest cells (NCCs), transplanted into chicken embryos, can differentiate into sensory, sympathetic, and parasympathetic neurons, as shown by the expression of neuronal subtype-specific and pan-neuronal markers, as well as into Schwann cells and satellite glia. In contrast, an immunopurified population of enteric neural precursors (ENPs) from the fetal gut can also generate neurons in all of these ganglia, but only expresses appropriate neuronal subtype markers in Remak's and associated pelvic parasympathetic ganglia. ENPs also appear restricted in the kinds of glia they can generate in comparison to NCCs. Thus ENPs have parasympathetic and presumably enteric capacities, but not sympathetic or sensory capacities. These results identify a new autonomic lineage restriction in the neural crest, and suggest that this restriction preceeds the choice between neuronal and glial fates.     

The Fine Structure of Nerve Cells and Fibers, Neuroglia, and Sheaths of the Ganglion Chain in the Cockroach (Periplaneta americana)

The abdominal nerve cord of Periplaneta americana was studied utilizing light and electron microscopes. In the nerve cells, delicate granules, similar to those probably responsible for cytoplasmic basophilia, are evenly distributed in "dark" cells and clumped in "light" cells. Neuroglial cells are stained metachromatically by cresyl violet. The neuroglial cells have many processes which ramify extensively and are enmeshed to form overlapping layers. These imbricated processes ensheath the nerve cells; the inner layer of the sheath penetrates into the neuron and is responsible for the appearance of the trophospongium of Holmgren. Nerve fibers are embedded within glial cells and surrounded by extensions of the plasma membrane similar to mesaxons. Depending on their size, two or several nerve fibers may share a single glial cell. Nerve fibers near their terminations on other nerve fibers contain particles and numerous, large mitochondria. The ganglion is ensheathed by a thick feltwork of connective tissue and perilemmal cells. The abdominal connective has a thinner connective tissue sheath which is without perilemmal cells. The nerve fibers and sheaths in the connective become thinner as they pass through ganglia.     

The fine structure of nerve cells and fibers,neuroglia, and sheaths of the ganglion chain in the cockroach (Periplaneta americana)

The abdominal nerve cord of Periplaneta americana was studied utilizing light and electron microscopes. In the nerve cells, delicate granules, similar to those probably responsible for cytoplasmic basophilia, are evenly distributed in "dark" cells and clumped in "light" cells. Neuroglial cells are stained metachromatically by cresyl violet. The neuroglial cells have many processes which ramify extensively and are enmeshed to form overlapping layers. These imbricated processes ensheath the nerve cells; the inner layer of the sheath penetrates into the neuron and is responsible for the appearance of the trophospongium of Holmgren. Nerve fibers are embedded within glial cells and surrounded by extensions of the plasma membrane similar to mesaxons. Depending on their size, two or several nerve fibers may share a single glial cell. Nerve fibers near their terminations on other nerve fibers contain particles and numerous, large mitochondria. The ganglion is ensheathed by a thick feltwork of connective tissue and perilemmal cells. The abdominal connective has a thinner connective tissue sheath which is without perilemmal cells. The nerve fibers and sheaths in the connective become thinner as they pass through ganglia.     

The demonstration of close nerve-Purkinje fibre contacts in false tendons of sheep heart

Summary An observation of intimate nerve-Purkinje fibre associations in false tendons of sheep heart is reported. Nerve bundles were observed in deep clefts of Purkinje fibres, in channels running between coupled Purkinje cells and embedded within Purkinje cells, as well as in the outer connective tissue sheath. Most nerve terminals in these areas were filled with small clear vesicles and a few large dense-cored vesicles. Only a few axons with many small dense-cored vesicles were observed.Intimate associations (separation, 60 to 90 nm) between the Purkinje cell and nerve varicosity were observed in the deep clefts. Similar close appositions were also present where nerves were embedded in Purkinje cells. In these cases the Purkinje cell enclosing the nerve bundle formed intercellular junctions with its own sarcolemma.Elaborate sarcolemmal folds with multi-vesicular bodies were also frequently observed near nerve bundles and varicosities. The identity of the transmitter is unknown although the nerves forming intimate associations with Purkinje cells have a morphology typical of cholinergic nerves.     

Fine structure of the receptors at the myotendinous junction of human extraocular muscles

The myotendinous junction of the human extraocular muscles was studied by electron microscopy. Some peculiar receptorial structures have been found in the majority of the samples examined. These structures are very small and consist of 1) the terminal portion of one muscle fibre, 2) the tendon into which it inserts and 3), within the tendon, a rich nerve arborization, whose branches are always very close to the muscle component. Only one discontinuous layer, made up of flat cells, which lack a basal lamina and often show pinocytotic vesicles, encapsules every musculo-tendinous complex. The tendinous component consists of amorphous ground substance of different electron density, of collagen and elastic fibres and is divided in compartments by ramified cells, which make an inner capsular-like covering to the nerve fibres. Three types of afferent nerve endings can be identified. One type is usually more frequent than the others, possesses a large number of neurotubules and neurofilaments and few mitochondria and is always surrounded by a Schwann cell which forms finger-like processes penetrating into the axoplasm. The second type is only partially enveloped by the Schwann cell. The axoplasm is devoid of neurotubules and contains few neurofilaments, several mitochondria and groups of small clear vesicles placed in the areas uncovered by the glial sheath. The third one is completely surrounded by the Schwann cell, but is devoid of neurotubules and neurofilaments and full of mitochondria. These morphological features correspond well with the probable role of these receptorial structures, which is to ensure very exact and precise ocular movements.     

Fine Structure of Subepithelial “Free” and Corpuscular Trigeminal Nerve Endings in Anterior Hard Palate of the Rat

Axonally transported protein labeled many trigeminal nerve endings in subepithelial regions of the anterior hard palate of the rat. Sensory endings were most numerous in the lamina propria near the tips of the palatal rugae where large connective tissue and epithelial papillae interdigitated. Two kinds of sensory ending were found there: “free” endings, and a variety of corpuscular endings. The “free” sensory endings consisted of bundles of unmyelinated axons separated from the connective tissue by relatively unspecialized Schwann cells covering part or all of their surface and a completely continuous basal lamina; they were commonly found running parallel to the epithelium or near corpuscular endings. The corpuscular sensory endings all had a specialized nerve form, specialized Schwann cells, and axonal fingers projecting into the corpuscular basal lamina or connective tissue. There were at least four distinct types of corpuscular ending: Ruffini-like endings were found among dense collagen bundles, and they had a flattened nerve ending with a flattened Schwann lamella on either side. Meissner endings had an ordered stack of flattened nerve terminals with flattened Schwann cells and much basal lamina within and around the corpuscle. Simple corpuscles were single nerve endings surrounded by several layers of concentric lamellar Schwann processes. Glomerular endings were found in lamina propria papillae or encircling epithelial papillae; they were a tangle of varied neural forms each of which had apposed flattened Schwann cells, and a layer of basal lamina of varied thickness. Fibroblasts often formed incomplete partitions around Meissner and simple corpuscles.

The axoplasm of all kinds of subepithelial sensory endings contained numerous mitochondria and vesicles, as well as occasional multivesicular bodies and lysosomes; the axoplasm of all endings was pale with few microtubules and neurofilaments. The specialized lamellar Schwann cells had much pinocytotic activity. Four kinds of junctions were found between the corpuscular sensory endings and the lamellar Schwann cells: (1) symmetric densities that resemble desmosomes; (2) asymmetric densities with either the neuronal or glial membrane more dense; (3) neural membrane densities adjacent to Schwann parallel inner and outer membrane densities; and (4) sites of apparent Schwann endocytosis associated with neural blebs. The “free” sensory endings only made occasional desmosome-like junctions with their Schwann cells.

These observations are discussed in relation to possible mechanosensory transduction mechanisms, with particular attention to axoplasmic structure, axonal fingers, and neural and nonneural cell associations.     

Transient involvement of enkephalins in both the sympathetic and parasympathetic innervations of the submandibular gland of rats

Summary A time course study with enkephalin(Enk)-like immunoreactivity has revealed that nerve fibers intensely immunoreactive for Enk-8 appeared transiently only during the postnatal week 2 and 4 within the acini as well as in the inter- and intralobular connective tissues of the submandibular gland of rats. At these stages numerous nerve fibers immunoreactive for tyrosine hydroxylase (TH) appeared also in the inter- and intralobular connective tissues and within the acini. Coincidently with these postnatal stages, abundant Enk-immunoreactive principal ganglion cells appeared in the superior cervical ganglion. These were not immunoreactive for neuropeptide tyrosine (NPY). A substantial number of Enk-immunoreactive ganglion cells were also present in the submandibular ganglia at these younger postnatal stages. Superior cervical ganglionectomy at these stages resulted in a marked decrease in number of the inter- and intralobular Enk-immunoreactive nerve fibers, a slight decrease in number of the intraacinar Enk-immunoreactive nerve fibers, and almost complete disappearance of intraglandular TH-immunoreactive nerve fibers. Immuno-electron-microscopic analysis revealed that Enk-immunoreactive nerve fibers in the submandibular gland were identified as electron-dense neuronal profiles enclosed by Schwann cells in the inter- and intralobular connective tissues and those directly apposed to secretory cells within the acini. They contained small clear vesicles mixed with some large granular vesicles. After postnatal week 6, no Enk-immunoreactive nerve fibers were detected in the submandibular gland, and no TH-immunoreactive nerve fibers were seen within the acini, while TH-immunoreactive nerve fibers remained numerous in the inter- and intralobular connective tissues. These findings indicate that both sympathetic and parasympathetic nerve fibers exhibit Enk-like immunoreactivity transiently during postnatal weeks 2 and 4. It is further indicated that the inter- and intralobular nerve fibers lose Enk-like immunoreactivity while the intraacinar fibers disappear at the adult stage.     

Membrane-bound neuregulin1 type III actively promotes Schwann cell differentiation of multipotent Progenitor cells

Many steps of peripheral glia development appear to be regulated by neuregulin1 (NRG1) signaling but the exact roles of the different NRG1 isoforms in these processes remain to be determined. While glial growth factor 2 (GGF2), a NRG1 type II isoform, is able to induce a satellite glial fate in neural crest stem cells, targeted mutations in mice have revealed a prominent role of NRG1 type III isoforms in supporting survival of Schwann cells at early developmental stages. Here, we investigated the role of NRG1 isoforms in the differentiation of Schwann cells from neural crest-derived progenitor cells. In multipotent cells isolated from dorsal root ganglia, soluble NRG1 isoforms do not promote Schwann cell features, whereas signaling by membrane-associated NRG1 type III induces the expression of the Schwann cell markers Oct-6/SCIP and S100 in neighboring cells, independent of survival. Thus, axon-bound NRG1 might actively promote both Schwann cell survival and differentiation.